1. Field of Endeavor
The present invention relates to a burner of a gas turbine; the invention also refers to a method for operating such a burner.
2. Brief Description of the Related Art
In particular, the present invention relates to a sequential combustion gas turbine, which includes a compressor for compressing a main air flow, a first burner for mixing a first fuel with the main air flow and generating a first mixture which is then combusted, a high pressure turbine where the combusted gasses are expanded, a second burner where a second fuel is injected into the gasses already expanded in the high pressure turbine to generate a second mixture which is then combusted, and a low pressure turbine where also these combusted gasses are expanded and are then discharged.
Specifically, a burner embodying principles of the present invention is the first burner of the sequential combustion gas turbine.
During normal operation gas turbines are typically fed with a gaseous fuel which is mixed with the air to generate the mixture to be combusted.
Nevertheless, for some reasons, such as interruptions of the gas service or gaseous fuel compressor problems, the gas may not be available for feeding the gas turbines.
For this reason, in order to prevent gas turbines from being stopped (they are usually used for electric power generation), gas turbines are also able to operate with a liquid fuel, such as oil, and can switch from gaseous fuel to liquid fuel, and vice versa, on-line.
U.S. Pat. No. 7,003,960 discloses a burner having a conical swirl generator provided at its lateral walls with apertures for tangentially feeding air and nozzles for injecting a gaseous fuel; this burner is also provided with a central lance for injecting a liquid fuel. In particular the lance is provided with a nozzle at its tip arranged to generate a conically propagating cloud of fuel within the swirl generator.
A further burner is disclosed in WO 03056241, which describes a burner with a conical swirl generator and downstream of it a mixing tube. The lateral walls of the conical swirl generator are provided with apertures for tangentially feeding air and nozzles for injecting a gaseous fuel. In addition, this burner has a lance which projects along its axis and is provided with nozzles at its lateral wall that are able to radially inject (i.e., in a direction perpendicular to the axis of the lance) a fuel.
The traditional burners described let low emissions be achieved and have the capability of being adapted to changes in ambient, fuel, and engine conditions, in particular at full load.
Nevertheless, during operation with liquid fuel (i.e. oil), burners must be fed with a mixture of oil and water (which is prepared upstream of the gas turbine) in order to prevent auto ignition of the droplets as soon as they go out from the nozzles.
Auto ignition would cause the liquid fuel droplets to burn in a zone of the burner close to the nozzles, where the droplets do not have enough air to correctly burn and before they have time to propagate towards zones richer in air. Thus auto ignition (with consequent combustion in an ambient poor of air) would cause high NOx emissions.
Water to be mixed with the liquid fuel must be previously purified and demineralised; this requires adapted plants and substantially involves high costs, in particular in regions (such as the Gulf region) where water is lacking.
In addition, existing burners have shown an operation that is not optimal, due to a poor and a not adaptable mixing quality of the fuel (both gaseous and liquid fuel) with the air.
The unadaptable mixing quality makes the burners unable to create (at partial and low load) a fuel rich central zone; this causes (at partial and low load) unstable flame, pulsations and low extinction limit.
In addition, poor mixing quality makes the NOx emissions increase at high load.